I think it depends on how your vet is assessing numbers. On an AlphaTrack or serum chemistry analyzer that's likely to be correct and vets tend to think in terms of pet specific meters rather than human meters. With TR, for a cat within a year of diagnosis, a dose reduction is given if numbers drop below 50 on a human meter (68 on a pet specific meter). If a cat is a longer term diabetic, reductions are given if the numbers are below 40. TR is based on research conducted at University of Queensland's vet school. If the details of the research weren't safe, the research would never get off the drawing board since the institutional review board would never approve the study. Likewise, a journal would never publish the protocol if it were unsafe especially since it's other vets who review a study for publication.
We've seen cats who have gone into remission and whose numbers, with no insulin on board, have numbers that are in the 40 - 60 range. It's where their BG levels naturally fall. They are fine. They may be more of an exception but it's still an example of how much variability there can be.
I also think it's a matter of how long a cat is staying in low numbers. Is it a matter of a few minutes or a few hours? I'd be nervous if it's the latter.